1. Field of the Invention
The present invention relates to a fixed subscriber unit and more particularly to a fixed subscriber unit for connecting a signal line telephone (SLT), which is connected thereto through wire, to a public network through radio.
2. Description of the Related Art
A fixed subscriber unit is connected to one or a plurality of signal line telephones (SLT) through wire and to a radio base station, which is connected to a public network, through radio to mediates speech or communication between the signal line telephone and another telephone which is connected to the public network.
As shown in FIG. 1, a conventional fixed subscriber unit is connected to one or a plurality of signal line telephones (SLT) (only one signal line telephone 31 shown in FIG. 1) through wire, and is supplied with electric power from an external power supply (AC/DC) 32. The fixed subscriber unit includes an antenna 42 for transmitting and receiving radio wave from a base station which is connected to a public network, and a radio unit 37 for demodulating a received signal and modulating a transmission signal together with a TDMA/TDD processing unit and CPU 36. The TDMA/TDD processing unit and CPU 36 controls the radio unit 37, a ringer driving circuit 40 and other circuits so that outgoing operation, incoming operation and communication are performed through radio between the signal line telephone 31 and another telephone which is connected to the public network. A speech circuit 38 is provided between the TDMA/TDD processing unit and CPU 36 and the signal line telephone 31. The speech circuit 38 converts two telephone lines for connecting the speech circuit 38 to the signal line telephone 31 into two transmission lines and two receiving lines (four lines in total) for connecting the speech circuit 38 to the radio unit 37, and controls formation of a DC loop between the signal line telephone 31 and the speech circuit 38 during the communication. A hooking detection circuit 43 for detecting hooking information from the signal line telephone 31 is provided between the speech circuit 38 and the signal line telephone 31. The ringer driving circuit 40 produces a ringing signal for ringing a ringer of the signal line telephone 31. A battery/charging circuit 41 connected to the external power supply (AC/DC) 32 supplies electric power to an electric power supply circuit 34 upon stoppage of supply of AC electric power such as power failure. The electric power supply circuit 34 supplies a DC current to the telephone lines for connecting the speech circuit 38 to the signal line telephone 31. First and second constant current circuits 391 and 392 provided in the telephone lines for connection of the speech circuit 38 and the signal line telephone 31 control the current supplied to the signal line telephone 31 to a constant current during the off-hook.
Operation of the fixed subscriber unit is now described. When there is an incoming call from an opposite telephone through the radio base station, the TDMA/TDD processing unit and CPU 36 which has received incoming information from the radio unit 37 controls the ringer driving circuit 40 to send a ringing signal to the signal line telephone (SLT) 31. When the signal line telephone 31 is off-hooked, the off-hook information is sent from the hooking detection circuit 43 to the TDMA/TDD processing unit and CPU 36. When the TDMA/TDD processing unit and CPU 36 receives the off-hook information, the TDMA/TDD processing unit and CPU 36 controls the speech circuit 38 to connect the signal line telephone 31 to the opposite telephone through the antenna 42, the radio unit 37 and the speech circuit 38. The electric power supply circuit 34 receives electric power from the external power supply (AC/DC) 32 to supply a voltage of 48 V to the signal line telephone 31 during the off-hook and to supply a DC current of 120 mA to the signal line telephone 31 during the communication.
As described above, the fixed subscriber unit can mediate communication between the single or the plurality of signal line telephones connected thereto through wire and the other telephone connected to the public network through radio.
In the conventional fixed subscriber unit, however, the voltage of 48 V is always supplied to the signal line telephone 31 and the current of 120 mA is also supplied thereto during the communication. Further, a voltage of 35 Vrms or more is supplied to the signal line telephone 31 during ringing of the ringer. Accordingly, it is necessary to supply such electric power to the signal line telephone 31 from the battery/charging circuit 41 during a power failure. Therefore, there is a problem that it is necessary to increase the number of battery cells and it is indispensable that the cost, weight and size of the fixed subscriber unit are increased due to the increased number of battery cells when a long-time operation of the fixed subscriber unit is to be ensured by the battery/charging circuit 41.